Conventional off-line switching power supplies employ transformers to isolate secondary circuits from primary utility line sources. Typically, rectifiers and filter capacitors combine to convert utility line AC to a high DC voltage (100-400 VDC) which, via the transformer, power switching circuitry, and secondary rectifier/filter circuits develops, from the transformer secondaries, lower DC voltages with higher current capability. In this environment, advantages of much smaller filter components and their resulting economies of weight and price can be realized by operating at higher switching frequencies.
Contemporary switching power supplies typically operate with switching frequencies in the vicinity of 20 kiloHertz (KHz). A significant packaging improvement can be realized if such supplies are adapted for operation at switching frequencies up to 1 megaHertz (MHZ). However, operation at this very high frequency (VHF) requires significant changes in transformer construction. For instance, the leakage inductance between primary and secondary windings or between secondary and tertiary windings must be substantially reduced if efficient power transfer is to occur. Minimization of leakage inductance demands that critical conductors be as physically intimate as possible. Other deleterious parasitic effects, such as skin effect, also become of primary concern as switching frequency increases.
Further complications result from geometry and uniformity constraints which tend to conflict with international safety standards that demand voltage isolation of up to 3750 VAC between line referenced primary circuits and secondary circuits to which personnel may be exposed. Other challenges from international standards appear in the form of requirements for conductive safety shields (often called screens) between primary and secondary windings as well as critical spacings of three to eight millimeters along surface paths between primary conductors and other conducting surfaces.
It is therefore desirable to make a power transformer capable of providing the economies of size, weight, and price afforded by operating in the VHF range of switching frequency, while respecting the physical and electrical safety requirements of primary to secondary electrical isolation.